This invention pertains to an electro-optical object-position-monitoring system, and more particularly to such a system which utilizes a tracking detector whose "rest" position, following tracking, is readable as an indication of the spatial location of the surface of a monitored object.
There are many instances, such as for example in a plywood mill, where it is desired to make an accurate measurement of the distance between a known location and a particular object, so as to locate the object (or more particularly the object's near surface) as precisely as possible in space. A preferred embodiment of the present invention is described herein in conjunction with a block-centering operation, wherein it is important to be able to position a log in a closely controlled position relative to a veneer peeler blade.
According to this embodiment a small-diameter laser beam is projected along one axis toward what might be thought of as a viewing zone, in which logs are expected to appear prior to a block-centering operation. "Looking," so-to-speak, along another and intersecting axis are a lens, and an optically sensitive receptor which looks for light from this beam which is reflected from a log's surface.
When the laser beam strikes the surface of a log, the lens focuses toward the receptor an image of the reflected light. The position of the center of such an image is directly interpretable to indicate the distance to the impinged log surface.
Employed as a receptor in one embodiment of the invention is a beam-splitting prism which divides light that it receives into two optical paths which extend toward two different photodetectors. The prism and photodetectors are mounted on a carriage which is movable back and forth along a predetermined path under the influence of a servo-drive device. The photodetectors are connected to two inputs in a comparator circuit which produces a DC output signal whenever the amount of light received by the two photodetectors differs. The polarity of this signal indicates which of the two photodetectors is receiving the greater amount of light. When the amount of light received by the two photodetectors is equal, no output signal is generated.
The output signal produced by the comparator circuit is fed to the servo-drive which responds by driving the carriage so as to shift the prism and photodetectors toward a location where illumination on the two photodetectors is equal. The condition of the servo-drive, with the carriage at rest, is monitored by a conventional rotary shaft encoder and output circuit which produces an output voltage whose magnitude is directly interpretable to indicate the position of the carriage. According to an alternate embodiment of the invention, the prism is replaced by two side-by-side disposed arrays of fiber-optic elements. The elements in each such array, at one end of the array, are splayed so as to place the elements' ends side-by-side in a row. From this end of the array, and progressing toward the other end, the elements merge into a bundle, the opposite end of which is aimed toward a photodetector. The splayed ends of the arrays are mounted for movement on a carriage like that mentioned above. The output terminals of the photodetectors connect as earlier mentioned to the inputs in a comparator circuit, and the remaining structure in this embodiment is duplicative of that contained in the first-outlined embodiment. Here also a servo-drive functions to shift the carriage to a position wherein the amount of light directed onto the two photodetectors is equal. The condition of the servo-drive, under such a circumstance, is readable as an indication of the position of the point of adjacency of the element ends in the two side-by-side arrays.
In both of the embodiments just generally outlined, positioning of the carriage to produce equal light reception by the two photodetectors causes, in the case of the prism, the front edge thereof, and in the case of the fiber-optic arrays, the junction therebetween, to reside substantially centrally on the spot of reflected light imaged by the lens. Accordingly, the "at rest" position of the carriage is interpretable to indicate the spatial location of the surface of a monitored log.
One of the important advantages offered by each of these modifications is simplicity, coupled with a high degree of accuracy.
These and various other objects and advantages which are attained by the invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying drawing.